Three-axis accelerometers and fabrication methods

ABSTRACT

MEMS accelerometers have a substrate, and a proof mass portion thereof which is separated from the substrate surrounding it by a gap. An electrically-conductive anchor is coupled to the proof mass, and a plurality of electrically-conductive suspension anus that are separated from the proof mass extend from the anchor and are coupled to the substrate surrounding the proof mass. A plurality of sense and actuation electrodes are separated from the proof mass by gaps and are coupled to processing electronics. The fabrication methods use deep reactive ion etch bulk micromachining and surface micromachining to form the proof mass, suspension arms and electrodes. The anchor, suspension arms and electrodes are made in the same process steps from the same electrically conductive material, which is different from the substrate material.

RELATED APPLICATION

The present application is a continuation of the U.S. patent applicationSer. No. 13/032,081, filed on Feb. 22, 2011, now U.S. Pat. No.8,173,470, issued on May 8, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/503,382, filed on Jul. 15, 2009, now U.S. Pat.No. 7,892,876, issued on Feb. 22, 2011, which is a divisional of U.S.patent application Ser. No. 11/431,168, filed on May 10, 2006, now U.S.Pat. No. 7,578,189, issued on Aug. 25, 2009, which are hereinincorporated by reference in their entirety for all purposes.

BACKGROUND

The present invention relates generally to three-axismicroelectromechanical systems (MEMS) accelerometers and fabricationmethods relating thereto.

Three-axis accelerometers have heretofore been developed for use withmotion control instrumentation, laptop computers, gaming consoles andcellular telephones, for example. Generally, such three-axisaccelerometers operate based upon either piezoresistive or capacitiveacceleration sensing. Capacitive three-axis accelerometers relate to thepresent invention and a variety of them are disclosed in various patentsand publications.

A number of three-axis accelerometers have been patented by KazuhiroOkada that use piezoresistive or capacitive acceleration sensing. U.S.Pat. No. 4,967,605 discloses a force detector that detects force usingresistance elements. The three detector uses “resistance elements havingthe piezo resistance effect” that “are formed on a single crystalsubstrate to connect a strain generative body having a supportingportion and a working portion thereto to allow the resistance elementsto produce a mechanical deformation on the basis of a displacement withrespect to the supporting portion of the working portion, thus toelectrically detect a force acting on the working portion.” It is statedin U.S. Pat. No. 4,967,605 that “When a force is applied to the workingportion of the force detector according to this invention, there occursa change in the electric resistance based on mechanical deformation bypiezo resistance effect, thus making it possible to electrically detectthe three applied.” While the force detector disclosed in U.S. Pat. No.4,967,605 (and those disclosed by others in U.S. Pat. Nos. 5,485,749 and6,683,358 B1 and US Patent Application No. US2005/0160814 A1) have astructure that is somewhat similar to the accelerometer disclosedherein, it is actually dissimilar, since the structure is configured toemploy piezoresistive elements and is made of a number of bondedsubstrates.

U.S. Pat. Nos. 5,406,848 and 6,716,253 issued to Okada, and U.S. Pat.No. 5,567,880 issued to Yokota, et. al., a paper entitled “Electrostaticservo system for multi-axis accelerometers” by Jono, et. al., a paperentitled “Three-axis capacitive accelerometer with uniform axialsensitivities” by Mineta, et. al., a paper entitled “Design andprocessing experiments of a new miniaturized capacitive triaxialaccelerometer” by Puers, et. al. and a paper entitled “Five-axis motionsensor with electrostatic drive and capacitive detection fabricated bysilicon bulk micromachining” by Watanabe, et. al., for example, disclosethree-axis acceleration detectors using capacitive sensing. Thesedetectors have multiple separated substrates with electrodes disposed onthem that are used to capacitively sense acceleration.

High volume three-axis accelerometer applications, in particularconsumer applications, are extremely performance, cost and sizesensitive. For a given set of performance requirements, cost must bereduced through minimizing chip (die) area, simplifying the fabricationprocess and using standard integrated circuit processes. Minimizing chiparea also minimizes the lateral dimensions (i.e., width and length) ofthe accelerometer chip. However, there are also increasing requirementsfor minimizing the thickness of the chip, for example for use in veryslim cell phones.

The prior art suffers from shortcomings that compromise reduction ofcost and size (chip thickness and area) while meeting performancerequirements. The prior art embodiments arrange the accelerometer proofmass, the suspension beams and the sense elements in such a way that oneor more of the following is compromised: chip size reductionoptimization, proof mass increase optimization, suspension beamcompliance optimization and/or fabrication process simplification. Forexample, all of the design embodiments are based on substrate bondingtechniques which increase the fabrication cost and chip thickness, ifnot also the chip area. In many of the design embodiments, the top ofthe proof mass and the suspension beams are arranged in the same plane,forcing compromises in increasing the proof mass, suspension beamcompliance and/or chip size. In some cases where the suspension beamsare place in a plane above or below the top or bottom plane of the proofmass to reduce chip area, the fabrication process is complicated andchip thickness is increased because of substrate bonding needs of thedesign embodiment.

It is also known to those skilled in the art that substrate bondingintroduces feature alignment and structural thickness inaccuracies thatare larger than those in techniques where all feature alignment iscarried out on the same substrate and structural film thicknesses aredetermined by film deposition. As a result, substrate bondingfabrication techniques often lead to device performance variations thatare larger than those resulting from single-substrate substratefabrication processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 is a top view of an exemplary three-axis MEMS accelerometer;

FIG. 1 a is a top view of an alternative embodiment of the three-axisMEMS accelerometer;

FIG. 2 is a cross-sectional view of the exemplary accelerometer shown inFIG. 1 taken along the lines 2-2;

FIG. 3 is a side view of the exemplary accelerometer shown in FIG. 1taken along the lines 3-3;

FIGS. 4 a and 4 b illustrate the operational principle for accelerationdetection using the accelerometer; and

FIGS. 5-14 illustrate steps of an exemplary process that may be used tofabricate the three-axis MEMS accelerometer shown in FIG. 1.

DETAILED DESCRIPTION

Disclosed herein is an exemplary three-axis microelectromechanicalsystems (MEMS) accelerometer 10 and an exemplary method 20 forfabricating the accelerometer 10. Referring to the drawing figures, FIG.1 is a top view of an exemplary three-axis MEMS accelerometer 10. FIG. 2is a cross-sectional view of the exemplary accelerometer 10 taken alongthe lines 2-2, and FIG. 3 is a side view of the exemplary accelerometer10 taken along the lines 3-3.

As is illustrated in FIGS. 1-3, the exemplary three-axis MEMSaccelerometer 10 comprises a substrate 11, which may comprise singlecrystal silicon, for example. The single crystal silicon substrate 11 ispreferably of very low resistivity to facilitate electrical contact andinterconnection. The substrate 11 may be made from a double-sidedpolished, 400 μm thick silicon wafer, for example. Use of commondouble-sided polished silicon substrates, for example in contrast tosilicon-on-insulator and bonded substrates, minimizes substrate costs.

A portion of the substrate 10 that comprises a proof mass 13 isseparated from an exterior support structure by a plurality of thinetched cavities 12. An electrically-conductive anchor 14 is coupled tothe top of the proof mass 13. A plurality of electrically-conductivetransverse suspension arms 15 or beams 15 (that form flexural springs)extend laterally from the anchor 14 beyond the lateral edges of theproof mass 13 to the exterior support structure where they terminate ata plurality of electrodes 16 (bond pads 16). The plurality ofelectrically-conductive transverse suspension arms 15 and bond pads 16may be made using surface micromachining techniques and are thus made ofa structural layer that is different from that forming the proof mass13. The proof mass 13 is preferably made using deep reactive ion etching(DRIE) of the substrate 11.

The plurality of electrically-conductive transverse suspension arms 15or beams 15 extend along x and y axes of the accelerometer 10, while theproof mass 13 extends from the anchor 14 along the z axis of theaccelerometer 10. The anchor 14, suspension arms 15 and bond pads 16 arefabricated in the same steps from the same structural layer and comprisethe same material, for example low-resistivity polysilicon of the samen- or p-type as the substrate 11. Alternatively, the anchor 14,suspension arms 15 and bond pads 16 may comprise a metal, such asnickel, for example.

A plurality of sense and actuation electrodes 17 are disposed above theproof mass 13 and are separated therefrom by gaps, and extend beyond thelateral edges of the proof mass 13 to the exterior support structurewhere they terminate at a plurality of electrodes 18 (bond pads 18). Thesense and actuation electrodes 17 and bond pads 18 are fabricated in thesame steps and from the same structural layer as the suspension arms 15,bond pads 16 and anchor 14, and therefore comprise the same material.The sense and actuation electrodes 17 operate using the principle ofcapacitive sensing and electrostatic actuation.

Each of the sense and actuation electrodes 17 and suspension arms 15 arecoupled by way of the bond pads 18, 16 to processing circuitry 19. Theprocessing circuitry 19 is configured to process electrical signals thatare sensed by electrodes 17 in response to movement of the proof mass13. Processing of the signals derived from electrodes 17 allowsdetermination of the acceleration components along the respective x, yand z axes of the accelerometer 10. Processing of electrical signals onelectrodes 17 to actuate a movement of the proof mass 13 in the x, y andz is used for force balance operation or for self-test andself-calibration functions. This processing is well known to thoseskilled in the accelerometer art. It is also known to those skilled inthe art that this sensing and actuation can be functionally andtemporally superimposed using the same electrodes 17.

FIGS. 4 a and 4 h illustrate the operational principle for accelerationdetection using the accelerometer 10. With regard to the accelerometer10 shown in FIGS. 4 a and 4 b, the gaps between the proof mass 13 andsurrounding substrate 11 are exaggerated as well as the distancesbetween the sense electrodes 17 and the proof mass 13, the movement ofthe proof mass 13 and bending of the suspension arms 15. The x, y and zaxes are shown at the upper left side of FIGS. 4 a and 4 b.

FIG. 4 a illustrates x-direction (or y-direction) acceleration. Theproof mass 13 moves in the x-direction (or y-direction) causing thetransverse suspension arms 15 or beams 15 to bend. The respectivedistances between top surface of the proof mass 13 and the senseelectrodes 17 change during acceleration which is capacitively sensed bythe sense electrodes 17 to produce respective electrical signals. Theseelectrical signals are processed accordingly by the processing circuitry19 to produce x-direction (and y-direction) acceleration data.Similarly, FIG. 4 b illustrates z-direction acceleration. In this case,the proof mass 13 moves closer to or away from the sense electrodes 17,which produces electrical signals that are processed by the processingcircuitry 19 to produce z-direction acceleration data. With respect toforce balance operation, or self-test and self-calibration, thedemonstrated movement of the proof-mass 13 in FIG. 4 a, for example, isinduced controllably by electrostatic actuation through applyingcontrolled voltages on the appropriate electrodes. This controllablyinduced movement is then sensed as described above, closing the controlloop for force balance operation, or self-test and self-calibration.

It should be clear to those skilled in the art that the x-y axesorientation may be rotated in the x-y plane for variations inorientations, geometries and locations of the suspension arms 15,electrodes 17 and bond pads 16, 18. For example, a 45 degree clockwiseor counter clockwise rotation of the x-y axes in the x-y plane couldhave the suspension arms 15 extending over the lateral diagonals of theproof mass 13 and the electrodes 17 extending from the substrate 11 overthe lateral center lines of the proof mass 13, wherein the electrodes 17have triangular geometries with bases at the substrate 11 and apexestoward the anchor 14. In some cases, the suspension beams 15 may bemodified, for example, to folded, perforated and slotted beams (andcombinations thereof), to adjust the compliance of the suspension beams15, reduce the effect of squeeze film damping on the proof mass 13movement speed and mitigate the effect of residual stresses from thesuspension beams 15 structural layer material. In some cases, electrodes17 may be further partitioned, and their geometries and locationsvaried, for more flexibility and functionality in sensing and actuation.Furthermore, perforations and/or slots may be incorporated in theelectrodes 17 to reduce the effect of squeeze film damping on the proofmass 13 movement speed and mitigate the effect of residual stresses fromthe electrodes 17 structural layer material. Such perforations and slotsin the electrodes 17 may be optimized to minimize any resultingdegradation in the mechanical stiffness and electrical performance ofelectrodes 17. These concepts and techniques are generally known andtheir utility should be clear to those skilled in the art.

Possible embodiment variations of the exemplary accelerometer 10 shouldalso be clear to those skilled in the art. For example, an inside-outversion (i.e., in the x-y plane), illustrated in FIG. 1 a, would havethe proof mass 13 symmetrically surrounding the interior substrate 11.Electrically-conductive anchors 14 would be coupled to the top of theproof mass 13. A plurality of electrically-conductive transversesuspension arms 15 extend laterally from the anchors 14 beyond thelateral inside edges of the proof mass 13 to the interior supportstructure where they terminate at bond pad 16. The sense and actuationelectrodes 17 would then be anchored to the interior substrate 11 andextend out onto the proof mass 13 surrounding the substrate 11. Anothervariation may comprise a combination of the exemplary accelerometer 10and the foregoing inside-out version, wherein the proof mass 13 asoriginally surrounded by the substrate 11 also surrounds an interiorportion of the substrate 11 as in the inside-out version. The suspensionbeams 15 would be arranged and anchored as in the inside-out version inFIG. 1 a, while the sense and actuation electrodes 17 would be arrangedand anchored as in the exemplary accelerometer 10 of FIG. 1. Bond pads16, 18 would be located correspondingly.

FIGS. 5-14 illustrate steps of an exemplary process 20 or method 20 thatmay be used to fabricate an exemplary three-axis MEMS accelerometer 10,such as is shown in FIG. 1. The exemplary fabrication process 20 ormethod 20 may be implemented as follows.

As is shown in FIG. 5, a nitride electrical isolation layer 21 isdeposited on both sides of a silicon substrate 11. The nitride isolationlayer 21 may be on the order of 2500 Angstroms thick. As shown in FIG.6, the nitride isolation layer 21 on the front surface of the substrate11 is patterned using photoresist 22 to define areas in which cavities23 are formed that subsequently allow release of the proof mass 13.These shallow cavities 23 are etched into the silicon substrate 11 usingthe nitride isolation layer 21 and photoresist 22 as a mask. Thecavities 23 are on the order of 1 μm deep. Although not shown, cavities23 may also be incorporated under the bond pads 18 and electricalinterconnects between electrodes 17 and bond pads 18 to reduce theparasitic capacitances associated with the substrate 11 as furtherdescribed below. The back side nitride isolation layer 21 is removedafter the cavities 23 are etched into the silicon substrate 11, followedby the removal of the photoresist 22 as is shown in FIG. 7.

As is shown in FIG. 8, the silicon substrate 11 is thermally oxidized toform a sacrificial oxide layer 25 that fills the cavities 23 by localoxidation of silicon to form a generally planar top surface. Asacrificial oxide layer 25 is also formed on the back side of thesubstrate 11 in the same step. The sacrificial oxide layer 25 that isformed may be on the order of 2 μm thick. Although not shown, thesacrificial oxide layer 25 would also fill those cavities 23 that mayhave been incorporated under the bond pads 18 and electricalinterconnects between electrodes 17 and bond pads 18 to reduce theparasitic capacitances associated with the substrate 11. A blanket dryetch using reactive ion etching of the top side of the structure isperformed to remove any oxide that may have been grown on the nitrideisolation layer 21.

As is shown in FIG. 9, an anchor hole 26 is etched through the nitrideisolation layer 21 to expose the underlying silicon substrate 11. Inthis same step, the nitride under the bond pads 16 may also be patternedto expose the underlying silicon substrate 11 if electrical connectionof the proof mass 13 to the substrate 11 through theelectrically-conductive suspension anus 15 is desired.

As is shown in FIG. 10, a low-resistivity polysilicon layer 27 isdeposited which coats the top and bottom sides of the structure andfills the anchor hole 26, forming the electrically-conductive anchor 14.Referring to FIG. 11, the polysilicon layer 27 on the front side of thestructure is patterned to define the suspension arms 15, sense andactuation electrodes 17, and bond pads 16, 18, and the back sidepolysilicon layer 27 is removed. Although not shown, in this step, theedges of the bond pads 18 and electrical interconnects betweenelectrodes 17 and bond pads 18 are patterned beyond the edges of thosecavities 23 that may have been incorporated under them to reduce theparasitic capacitances associated with the substrate 11. As a result,along the entire periphery of those cavities 23, the polysilicon layer27 directly contacts the nitride electrical isolation layer 21, sealingthe oxide sacrificial layer 25 inside those cavities 23 from beingetched when the proof mass 13 is subsequently released. Because thesacrificial oxide layer 25 is thicker than the nitride isolation layer21 and also has a lower permittivity, the substrate parasiticcapacitances associated with bond pads 18 and electrical interconnectsbetween electrodes 17 and bond pads 18 are substantially reduced by thistechnique without adding fabrication process steps to accomplish thesame. As known by those skilled in the art, reducing these substrateparasitic capacitances enhances sensor performance.

As is shown in FIG. 12, the sacrificial oxide layer 25 on the back sideof the structure is patterned using a mask layer 28 to define the thinetched cavities 12 for forming the proof mass 13 and provide“auto-dieing” lines. The latter removes risk of damage to themicromechanical structures during traditional dicing using a saw as wellas eliminating this cost.

As is shown in FIG. 13, a deep reactive ion etch is performed to etchthrough the back side of the silicon substrate 11 to expose thesacrificial oxide layer 25 disposed beneath the polysilicon layer 27 onthe front side of the structure. The deep reactive ion etch defines theouter edge of proof mass 13.

As is shown in FIG. 14, the proof mass 13 is released from thesurrounding silicon substrate 11 and polysilicon layer 27 (suspensionbeams 15 and sense and actuation electrodes 17) by dissolving thesacrificial oxide layer 25. This finalizes fabrication of theaccelerometer 10.

After fabrication, the accelerometer 10 is typically mounted on apackaging substrate. In order to not obstruct the movement of the proofmass 13 by the mating surface of the packaging substrate, a provision ismade to recess the proof mass 13 slightly (e.g., 2 μm) from the matingsurface of the packaging substrate. Thus, the packaging substrate willhave a cavity in it to allow separation of the proof mass 13. If thiscavity is not provided in the packaging substrate, then an additionalpattern/etch step on the back side of the substrate 11 is needed, whichmay be performed between the steps shown in FIGS. 7 and 8, to create therecess in the substrate 11 on the bottom side where the proof mass 13will reside. In this case, the proof mass 13 is recessed from the bottomsurface of substrate 11 surrounding it. Regardless, the packagingsubstrate may also incorporate sensing and actuation electrodes underthe proof mass 13, within the cavity, for detection and actuation of theproof mass 13 movement.

It should be clear to those skilled in the art that the steps shown anddescribed with reference to FIGS. 12 and 13 may be used to also patternthe substrate 11 into physically (and therefore electrically) separateregions anchoring suspension beams 15/bond pads 16, supporting sense andactuation electrodes 17/bond pads 18 and also implementing verticalsense and actuation electrodes from columnar features etched into thesubstrate 11 and separated from the proof mass 13 by thin etchedcavities 12.

The three-axis micro electromechanical systems (MEMS) accelerometer 10disclosed above has a number of features that distinguish it from theprior art. The disclosed embodiment eliminates the need for waferbonding in realizing the proof mass 13, suspension arms 15,sense/actuation electrodes 17 and anchor 14. This leads to processsimplification and chip size reduction (importantly, including chipthickness).

The anchor 14, suspension arms 15, sense/actuation electrodes 17 andbond pads 16, 18 are made simultaneously from the sameelectrically-conductive (i.e., low-resistivity polysilicon) material,which also provides the necessary electrical contact and interconnectprovisions for the sensor. Within the real estate required for the proofmass 13 size, the suspension arms 15, sense/actuation electrodes 17 andanchor 14 are implemented, eliminating real estate overhead for thesame.

The single-substrate design allows for accurate feature alignment,eliminating the related sensitivity imbalances of the proof mass 13 andsuspension arms 15 otherwise resulting from such alignment inaccuracies,for example in substrate bonding approaches. The overall design based ona single substrate 11 is more compatible with integrated circuitfabrication processing and therefore less costly in manufacturing. Italso lends itself to integration of the sensor with the necessaryinterface electronics 19 on the same substrate 11, when doing so isdesired.

The thickness of the suspension arms 15 is determined by filmdeposition, which can be controlled accurately in manufacturing,minimizing variations of compliance of the suspension arms 15 (andtherefore sensor mechanical element sensitivity). The surfacemicromachining enabled embodiment allows anchor 14 to be very small,approximating a pivot point in order to enhance the sensitivity of themechanical structure to acceleration, reducing the required sensor size.Once affixed to a package substrate from the bottom side of sensorsubstrate 11, movement of the proof mass 13 is constrained on all itssides, enabling good mechanical stop against undesirable shock.

Deep reactive ion etching enables separating the proof mass 13 from thesubstrate 11 with thin etched cavities 12 that are utilized for shockprotection and also enable real estate efficiency. The local oxidationof silicon technique, to create the sacrificial oxide layer and agenerally planar surface, eliminates surface topography to allow forhigh quality pattern definition of the suspension arms 15 andsense/actuation electrodes 17, i.e., elements whose geometry is criticalto the device performance. It also eliminates steps where these elementsare anchored to the proof mass 13 and substrate 11 (as applicable). Itis known in the art that these steps are weak mechanical points ofpremature failure, but are often inherent to the design and fabricationprocesses that have been employed.

Thus, three-axis accelerometers and fabrication methods relating theretohave been disclosed. It is to be understood that the above-describedembodiments are merely illustrative of some of the many specificembodiments that represent applications of the principles discussedabove. Clearly, numerous and other arrangements can be readily devisedby those skilled in the art without departing from the scope of theinvention.

1. A method of fabricating an accelerometer, comprising: forming apatterned isolation layer disposed on a front side of a substrate;forming a sacrificial layer on the substrate; removing the sacrificiallayer on the isolation layer; forming a patterned low-resistivitystructural layer on the front side of the substrate to define suspensionarms, sense and actuation electrodes, and bond pads; patterning thesacrificial layer on a back side of the substrate; etching the back sideof the substrate to expose the sacrificial layer disposed beneath thelow-resistivity structural layer on the front side of the substrate andto define outer edge of a proof mass; and releasing the proof mass bydissolving the sacrificial layer.
 2. A method of fabricating anaccelerometer, comprising: providing a substrate; forming an isolationlayer on the substrate; patterning the isolation layer disposed on afront side of the substrate; forming a sacrificial layer on thesubstrate; removing the sacrificial layer on the isolation layer;etching anchor holes and bond pad contact windows through the isolationlayer to expose the underlying substrate; forming a low-resistivitystructural layer on the substrate; patterning the low-resistivitystructural layer on the front side of the substrate to define suspensionarms, sense and actuation electrodes, and bond pads; patterning thesacrificial layer on a back side of the substrate if such is formed;etching the back side of the substrate to expose the sacrificial layerdisposed beneath the low-resistivity structural layer on the front sideof the substrate and to define outer edge of a proof mass; and releasingthe proof mass by dissolving the sacrificial layer.
 3. The methodrecited in claim 2 wherein the sacrificial layer comprises oxide, andetching the back side of the substrate comprises dry reactive ionetching.
 4. The method recited in claim 2 wherein the substratecomprises a semiconductor substrate.
 5. The method recited in claim 2wherein the substrate comprises a silicon substrate.
 6. The methodrecited in claim 2 wherein the substrate comprises asemiconductor-on-insulator substrate.
 7. The method recited in claim 2wherein the substrate comprises a metal in which deep trenches may befabricated by reactive ion etching.
 8. The method recited in claim 2wherein the structural layer comprises a semiconductor layer.
 9. Themethod recited in claim 2 wherein the structural layer comprises dopedpolysilicon.
 10. The method recited in claim 2 wherein the structurallayer comprises metal.
 11. The method recited in claim 2 wherein thesacrificial layer comprises oxide, and etching the back side of thesubstrate comprises dry reactive ion etching.
 12. The method recited inclaim 2 wherein the substrate comprises a semiconductor substrate. 13.The method recited in claim 2 wherein the substrate comprises a siliconsubstrate.
 14. The method recited in claim 2 wherein the substratecomprises a semiconductor-on-insulator substrate.
 15. The method recitedin claim 2 wherein the substrate comprises a metal in which deeptrenches may be fabricated by reactive ion etching.
 16. The methodrecited in claim 2 wherein the structural layer comprises asemiconductor layer.
 17. The method recited in claim 2 wherein thestructural layer comprises doped polysilicon.
 18. The method recited inclaim 2 wherein the structural layer comprises metal.